What are these, by the way? That would answer much of my OP question.
To go with the simplest answer, it is possible to gauge what the response would have been from the US with quotes from wikipedia:
The Truman administration was caught ill prepared and at a crossroads. Before the invasion, Korea was not included in the strategic Asian Defense Perimeter outlined by Secretary of State Dean Acheson.[115] Military strategists were more concerned with the security of Europe against the Soviet Union than East Asia. At the same time, the Administration was worried that a war in Korea could quickly widen into another world war should the Chinese or Soviets decide to get involved as well.
A major consideration was the possible Soviet reaction in the event that the US intervened. The Truman administration was fretful that a war in Korea was a diversionary assault that would escalate to a general war in Europe once the United States committed in Korea. At the same time, "[t]here was no suggestion from anyone that the United Nations or the United States could back away from [the conflict]".[117] Truman believed if aggression went unchecked a chain reaction would be initiated that would marginalize the United Nations and encourage Communist aggression elsewhere. The UN Security Council approved the use of force to help the South Koreans and the US immediately began using what air and naval forces that were in the area to that end. The Administration still refrained from committing on the ground because some advisers believed the North Koreans could be stopped by air and naval power alone.[118]
From these two excerpts alone the following could be identified:
- The United States was not prepared to engage in a war as large as the last one;
- Thus, the United States was keen in deescalating the situation, either by stopping it or ignoring it completely;
- The Communist threat was closely felt, along with the possibility of one conflict leading to a larger one;
- Korea was never a valuable asset to the United States, and the intervention was ultimately for keeping Japan safe.
Thus the following two conclusions could be drawn:
- The decisive question was whether or not Japan, along with other island states of the Far East, would be safe from Communism;
- There were a variety of options for the United States, although the administration was less than willing to escalate the conflict.
From this it can be concluded that with the fall of Korea the worst form of escalation would in quite parallel that of the Iron Curtain in Europe(although significantly less so, since this "Curtain" in Asia would be the wide ocean). And if the border remains calm enough the situation would be further "deescalated" - seeing how much a lack of mainland aggression there was against Taiwan IOTL, Korean aggression against Japan would be much less. Furthermore considering what we see in West Germany IOTL during the Cold War a remilitarisation of Japan TTL does not need to be expected.
What would the reaction within the Japanese government be to the fall of Korea? And what about Mao, what would be his government's next steps after Communist victory in Korea?
the Rhee government was planning to move a mass of South Koreans into Japan, specifically the prefecture Yamaguchi; estimated number of refugees that could be logistically handled was 60,000. (There also was an American plan to move 600,000 Koreans to West Samoa and establish a "New Korea".) For Mao, his main issue would be to catch up in industrialisation with the West, and seeing how Kim was left alone with his thing IOTL I doubt there would be much intervention, if at all. An interesting note on Kim - if Korea actually became reunified under him, his power would actually become severely undermined as the south actually had a stronger Communist base, one that has matured over the years under colonial rule. A saner KWP can be expected, might even be neutral during the Cold War.